Skin tests for allergic disorders were first described in 1867 and quickly evolved into the scratch test, which was initially used to confirm the diagnosis of food allergy in children. Methods used for allergy skin testing may be performed using either prick or puncture (percutaneous) or intradermal (intracutaneous) techniques. In clinical practice, the scratch has given rise to the prick or puncture test, and in some cases if the results are interpreted as negative, it is followed by the intradermal test. Intradermal testing is more sensitive than prick or puncture testing, and as a result, the extract for prick or puncture testing must be at least 1,000-fold more concentrated to achieve a similar level of sensitivity. Although intradermal may be more reproducible than prick or puncture testing, there are many factors that favor the routine use of the prick or puncture test for allergy testing. These factors include economy of time, patient comfort, and safety.
Allergy skin testing is minimally invasive and when performed correctly has good reproducibility. It is also preferred because the test results are available within minutes of the test application, enabling the physician to quickly devise an appropriate treatment plan on the initial consultation. Skin testing is easily quantifiable and can allow the evaluation of multiple allergens in a single session. There are a variety of factors that can influence or confound the allergy skin test procedure. These factors include type of skin testing, applicator device used, placement of tests (location and adjacent testing), the quality of the extracts used, and the potential confounding effects of medication. The technique of the technician administering the skin test can also directly influence the results.
Prick or puncture (percutaneous) tests are performed with either a single or multi-test applicator device. The applicator is dipped into a tray containing wells that have been filled by the clinician with the allergen extracts. Each well is typically filled with approximately 500 or more microliters of the allergen extract and there may be as many as 48 or more individual wells each containing a different extract. The technician dips the applicator into the tray wells to place a consistent amount of allergen extract on the applicator. The applicator is pressed against the skin and punctures the skin approximately one millimeter and administers the extract by contact. It is important that the technician achieve consistency in the administering the extract at each individual site. The tray containing the extracts may be used to test as many as twenty five subjects before it is refilled. Proper clinical handling procedure is to cover and place the tray containing the unused extracts in refrigerated storage in between testing procedures to prevent loss of potency, contamination and cross contamination of the extracts.